[ECDH API change] Allow pass arbitrary data to hash function

include/secp256k1_ecdh.h

@@ -13,11 +13,13 @@ extern "C" {
  *  Out:    output:     pointer to an array to be filled by the function
  *  In:     x:          pointer to a 32-byte x coordinate
  *          y:          pointer to a 32-byte y coordinate
+ *          data:       Arbitrary data pointer that is passed through
  */
 typedef int (*secp256k1_ecdh_hash_function)(
   unsigned char *output,
   const unsigned char *x,
-  const unsigned char *y
+  const unsigned char *y,
+  void *data
 );
 
 /** An implementation of SHA256 hash function that applies to compressed public key. */
@@ -35,13 +37,15 @@ SECP256K1_API extern const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_func
  *                       initialized public key
  *           privkey:    a 32-byte scalar with which to multiply the point
  *           hashfp:     pointer to a hash function. If NULL, secp256k1_ecdh_hash_function_sha256 is used
+ *           data:       Arbitrary data pointer that is passed through
  */
 SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ecdh(
   const secp256k1_context* ctx,
   unsigned char *output,
   const secp256k1_pubkey *pubkey,
   const unsigned char *privkey,
-  secp256k1_ecdh_hash_function hashfp
+  secp256k1_ecdh_hash_function hashfp,
+  void *data
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
 
 #ifdef __cplusplus